There’s no doubt about the existence of acid rain and holes in the ozone layer.

There’s no doubt that the Arctic and Antarctic ice shelves have shrunk by 23 percent.

There’s no doubt that man-produced carbon dioxide has exponentially aggravated the greenhouse effect.

Irrefutable scientific data and photographic evidence now confirm all of the above. But there was a time when they didn’t.

What do you do when such data and evidence debunk your ideological beliefs? When there’s truly no doubt?

You sow it.

That’s the premise and political tactic that documentary director Robert Kenner (Oscar-nominated for “Food Inc.” in 2009) explores in “Merchants of Doubt,” an expose of the pundits-for-hire who pose as scientific authorities on such issues as toxic chemicals, pharmaceuticals and climate change.

Their genesis dates from the 1950s with the initial full-throated (pardon the adjective) defense of tobacco: “Nicotine is not addictive and smoking doesn’t cause cancer.” As research data showed otherwise, the defense modulated to: “It is not known whether smoking causes cancer” and then “There is no conclusive proof.”

That approach — backed by a billion-dollar public relations campaign to promote it — staved off government action/regulation for the better part of half a century, while other toxic industries took notice: If it worked for tobacco, why not for us?

The playbook: Deny the problem, call for more evidence, cherry-pick data, create front groups to raise concerns about threatened freedoms, hire media-friendly “experts” to promulgate disinformation and mislead the public (an easy-enough task).

Rather than develop a self-extinguishing cigarette, for example, the tobacco and chemical industries joined to fund “Citizens for Fire Safety,” a front group that paid doctors tens of thousands of dollars to invent crib-fire horror stories for congressional committees on behalf of the world’s three largest flame-retardant companies.

The Big Issue nowadays, of course, is climate change and the industry-funded campaigns to convince Americans that global warming is “a hoax” lacking scientific consensus. In fact, the science is conclusive. Don’t take it from me or filmmaker Kenner. Take it from George H.W. Bush: “Global warming is real, man-made and potentially catastrophic.” Newt Gingrich, John Boehner and Mitt Romney all said the same — until the Tea Party’s rise in 2008. Now they say, “We don’t know....”

Bottom line: You don’t need to prove or disprove anything to muck up the issue. You just need to confuse. You don’t have to win the debate — you just have to keep it going. A classic example, Mr. Kenner argues, is the so-called “Oregon Petition,” allegedly signed by 31,000 scientists and engineers denying climate change. Although long since discredited, it is endlessly cited and perpetuated on the Internet by the right-wing blogosphere and conservative think tanks —- such as Americans for Prosperity and the Cato Institute — funded by the Koch brothers.

America's spin factory is full of professional fabricators and obfuscators on both sides of the political fence, who have muddled the climate-change debate. But Mr. Kenner says it’s the right, rather than the left, that has been remarkably effective at creating scientific controversy where none exists.

In any case, regardless of left or right, you can't “reason” people out of a position they didn't reason themselves into, and most people are less interested in “the truth” than in confirming their predisposed biases. There’s a debate, all right, but it’s not about science, it’s about politics.

Mr. Kenner’s use of magician Jamy Ian Swiss as a framing device for his film is a bit too precious, although the intended parallel is apt: The magician’s techniques of distraction and confusion are similar to the global-warming spinmeister’s.

Based on a revelatory investigative book by Naomi Oreskes and Erik M. Conway, this documentary’s slick production values boost it above Michael Moore’s in-your-face docs but leave Mr. Kenner open to similar over-polemical charges. Not to mention his over-emotional ending. He’s preaching (largely) to his own choir. But it is fascinating as well as factual.

Most fascinating factoid, for me, in “Merchants of Doubt” is that the Arctic is warmer now than it has been in 40,000 years. Which relates to an irony revealed in the film: Exxon-Mobil was engaged in new drilling there — in a joint operation with Russia — using the latest sophisticated climate-change data, even as they’re denying it!